This invention relates to electroluminescent (EL) lamps and, in particular, to producing a color shift in the light emitted by an EL phosphor without significantly affecting the life of the lamp.
An EL panel is essentially a capacitor having a dielectric layer between two conductive electrodes, one of which is a transparent metal layer, such as indium tin oxide (ITO). The dielectric layer includes a copper doped ZnS phosphor powder or there is a separate layer of phosphor powder adjacent the dielectric layer. The phosphor powder radiates light in the presence of a strong electric field, using very little current. Because an EL lamp is a capacitor, alternating current must be applied to the electrodes to cause the phosphor to glow, otherwise the lamp charges to the applied voltage, current through the lamp ceases, and the lamp stops producing light.
EL phosphor particles are zinc sulfide-based materials, commonly including one or more compounds such as copper sulfide (Cu.sub.2 S), zinc selenide (ZnSe), and cadmium sulfide (CdS) in solid solution within the zinc sulfide crystal structure or as second phases or domains within the particle structure. EL phosphors commonly contain moderate amounts of other materials such as dopants, e.g., bromine, chlorine, manganese, silver, etc., as color centers, as activators, or to modify defects in the particle lattice to modify properties of the phosphor as desired. A copper-activated zinc sulfide phosphor produces blue and green light under an applied electric field and a copper/manganese-activated zinc sulfide produces orange light under an applied electric field. Together, the phosphors produce white light under an applied electric field.
Cu.sub.x S is a p-type semiconductor whereas ZnS:Cu,h (h=Cl, Br, I) is an n-type semiconductor. Needles of Cu.sub.x S deform an applied electric field, causing the local field strength to be as much as one thousand times higher than the average field strength. The color of the resulting light emission is determined by the doping levels. Although understood in principle, the luminance of an EL phosphor particle is not understood in detail. The luminance of the phosphor degrades with time and usage, more so if the phosphor is exposed to moisture or high frequency (greater than 1,000 hertz) alternating current.
In portable electronic devices, automotive displays, and other applications where the power source is a low voltage battery, an EL lamp is powered by an inverter that converts direct current into alternating current. In order for an EL lamp to glow sufficiently, a peak-to-peak voltage in excess of about one hundred and twenty volts is necessary. The actual voltage depends on the construction of the lamp and, in particular, on the field strength within the phosphor powder. The efficiency of the inverter affects battery life and lamp brightness.
The frequency of the alternating current through an EL lamp affects the life of the EL lamp, with frequencies between 200 hertz and 1,000 hertz being preferred. Ionic migration occurs in the phosphor at frequencies below 200 hertz. Above 1,000 hertz, the life of the phosphor decreases at a rate that is inversely proportional to frequency.
Because EL lamps provide uniform luminance and consume very little power, there is a great demand for EL lamps in displays. There is also a great demand for a variety of colors, which is difficult to meet from a limited number of phosphors. The color of a phosphor is a quantum mechanical phenomenon which, by definition, does not provide a continuous spectrum of colors. Mixing different phosphors, "cascading" phosphors (using the light from a phosphor to stimulate a fluorescent dye), and filtering are three of several techniques known in the art for obtaining colors other than the strongest emission band of a particular phosphor.
Any manipulation of a phosphor involves tradeoffs. Typically, various colors are obtained at the expense of brightness. Often, the life (time to half brightness) of a phosphor is also reduced. It is known that the color of a blue-green phosphor shifts toward blue as the drive frequency increases and the life of the phosphor decreases with increasing frequency; e.g. time to half brightness at 4,000 hertz is approximately one fourth the time at 1,000 hertz.
In view of the foregoing, it is therefore an object of the invention to provide a blue EL lamp having longer life and higher luminance than blue EL lamps of the prior art.
Another object of the invention is to provide an apparatus and method for varying the colors of EL lamps without substantially changing life or luminance.
A further object of the invention is to provide a technique for shifting the color emitted by an EL phosphor or phosphor mixture.
Another object of the invention is to provide a technique for shifting the color of light emitted from cascaded EL phosphors.
A further object of the invention is to improve the efficiency of battery powered inverters for EL lamps.